House Rules: Warlock Pact Magic

Warlocks (and potentially other characters--see the Invoke Patron section below), have the ability to call upon their patrons to boost their powers, with the risk of being corrupted by the touch of extraplanar energies.

Boost Spell

First, a warlock may call upon the favor of his patron to grant advantage on a spell attack roll, or disadvantage on an opponents spell save. However, when he does so, the warlock must make a DC 12 Constitution save, with an increased difficulty of +2 for each extra spell boost per day (i.e. DC 14 for the second boost, 16 for the third, 18 for the fourth, and so on). When failed, she must roll on the Invoke Patron Corruption table and take the effect.

Invoke Patron
In addition to the ability to have a patron boost their magical power, all Warlocks have access to the Invoke Patron ability, whereby they call upon their patron to have a direct (and luckily for reality, brief) influence on the material plane.

Non-warlock characters may also gain the ability to invoke otherworldly patrons, typically through special boons or favors from an entity. However, since invoking often simply involves calling out a patron's name for help, if a character learns a powerful being's name, there is always chance he may invoke that being's help even without a prior agreement. In some cases, powerful entities may have secret True names in addition to the regular names or monikers they go by; if someone somehow learns the True name of such an entity, invocation is nearly guaranteed (possibly even with a lesser chance of otherworldly corruption).

A patron may be invoked for nearly any reason, though as the consequences are dire, the wisest of warlocks (or at least those with the highest survivability rate) only risk invoking a patron as a last resort.

Invoke Patron generally works as a wish spell, though more unpredictable, as other spells may not be specifically duplicated, and with a roll on the invoke patron corruption table instead of the stress effects listed on the spell. The effects generally manifest in some way related to the patron himself. For example, a demonic patron’s manifestation might involve smoke, sulphur, and the screams of the damned; the Lord of the North Wind’s manifestation might involve a sudden snowstorm that freezes enemies solid; and a sylvan fey patron’s invocation might cause a swarm of mice, squirrels, and hares to erupt from a nearby tree, engulf enemies, and leave nothing but bone behind.

Invoke Patron comes with a price. For each Invocation a warlock must roll on the corruption table and take the result. Depending on the size and type of the invocation, the DM may allow a Con save to make the corruption temporary, but any second and on invocations per day get much less benefit of the doubt. If a warlock invokes for an effect directly benefitting another person, such as to heal a bleeding out companion, make him grow to three times his size, or bring him back to life there is a 50% chance the affected target must also roll on the corruption table.

Additionally, for each invocation within a given week, or permanently in the same location, there is a cumulative 10% chance of a potentially permanent Incursion centered on the invoking warlock or in that location. For example, the first time he invokes his patron, there is a 10% chance of an Incursion centered on the invoker; the second time within the same week there is a 20% chance, the third time a 30% chance, and so on. Locations are permanently tainted by the merest hint of a patron’s presence on the material plane. The chance of an Incursion never decreases for an area where a patron has been invoked, so for every time a patron is invoked in the same general area, the chance of a permanent taint on reality goes up 10%. It is no wonder then that warlocks are feared, shunned, and subject to lynchings; mad warlocks left unchecked run the risk of leaving the world wide open to influence from otherworldly entities with goals ranging from benign, to mysterious, to malevolent. If an Incursion takes place, roll on the Incursion table to see how the area where the Incursion takes place is effected.

Invoke Patron Corruption Effects
I'm not going to give everything away, the results are proprietary! But here's an overview of the basics.

1-10 Your patron must be bored with you, or maybe it’s just busy. Or not listening. Or maybe it likes to see you suffer. Whatever the case, nothing happens.

11-25 Your patron is displeased that you’ve bothered it. Your request goes unanswered, but roll again and take the corruption effect.

26-40 Some pretty messed up stuff happens to you, typically involving a permanent decrease of an ability score and/or worse effects. A limb might shrivel, or part of your brain leak out your nose, or something like that.

41-85 Some other unpleasant stuff happens to you, typically with some negative aspects, such as penalties to interactions with "civilized" society (i.e. those afraid of warlock corruption), but possibly (though for sure not definitely) with some benefits as well; typically the better the higher you roll. For example, you might grow an extra eye somewhere that makes you pretty freaky looking, but that eye might have darkvision, or let you see out the back of your head.

86-99 Roll again, re-rolling anything 40 or below. You gain the corruption effect, but can remove it for as long as you retain concentration.

100 Your patron is pleased with you! You suffer no negative repercussions, and in fact you may either remove a prior corruption, or have one of your ability scores permanently increased by 1. Roll a d6: (1) Strength; (2) Dexterity; (3) Constitution; (4) Wisdom; (5) Intelligence; (6) Charisma.